CODEWITHSUNDEEP
cshellposixbash
Danyel
Danyel

Reputation: 2250

C: List aliases of current shell (not subshell!)

I'm trying to find all aliases that the current shell possesses (in a C program). I've tried system("alias"), popen("alias", "r") and execvp("alias", ...), the latter of which doesn't work at all (because alias is a shell-specific command) and the first two of which run that command in a subshell (which is sh and not bash) -> there, aliases are disabled because they are defined in my ~/.bashrc. Executing bash and reading the output of alias isn't possible either because bash will only go to the alias definitions if it is in interactive mode.

If I do run bash in interactive mode, I get a huge delay time and a prompt output that I want to avoid.

Basically, what I want, is to have similar behaviour as time(1). It looks up current aliases without even executing any command! (it will fork only once, that is, for the passed command)

Crawling the Internet was to no avail.

Question: How do I look up all aliases in the current shell? Will there be any portability issues? If yes, how to avoid them?

Regards.

Upvotes: 1

Views: 213

Answers (2)

Marcelo Cantos
Marcelo Cantos

Reputation: 185852

I just downloaded and built GNU time on my Mac. To my surprise and chagrin, it doesn't read aliases from the parent bash, whereas the built-in time does.

$ alias frodo=ls
$ ./time frodo
./time: cannot run frodo: No such file or directory
Command exited with non-zero status 127
0.00user 0.00system 0:00.00elapsed ?%CPU (0avgtext+0avgdata 819200maxresident)k
0inputs+0outputs (0major+59minor)pagefaults 0swaps
$ time frodo
AUTHORS    INSTALL      Makefile.in  config.cache    configure*    error.o   getopt.o   getpagesize.h  mkinstalldirs*  resuse.h  stamp-vti    time.c     time.texi  version.texi
COPYING    Makefile     NEWS         config.log      configure.in  getopt.c  getopt1.c  install-sh*    port.h          resuse.o  texinfo.tex  time.info  version.c  wait.h
ChangeLog  Makefile.am  README       config.status*  error.c       getopt.h  getopt1.o  mdate-sh*      resuse.c        stamp-v   time*        time.o     version.o

real    0m0.005s
user    0m0.002s
sys 0m0.002s
$

I suspect there's something undocumented is passing under the table to help the builtin time see aliases (or BSD provides something that GNU time won't or can't make use of).

EDIT: Prompted by Tom Tanner's answer (+1 Tom), I just realised that time frodo doesn't invoke /usr/bin/time (as suggested by which time), and that explicitly running /usr/bin/time frodo also fails, so it must be that entering time by itself invokes a bash builtin command.

Upvotes: 2

Tom Tanner
Tom Tanner

Reputation: 9354

You can't. time is a built-in and it can access the aliases which are stored internally to the shell instance that is running. if you need to work out what the shell will execute, you need to run which or something similar.

time isn't doing anything clever or secret. It's just a prefix to the command to make the shell print out some timing information.

Upvotes: 2

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